It is known that a relevant problem to be overcome in a gas-phase polymerization process is the formation of polymer agglomerates, which can build up in various places, such as the polymerization reactor and the lines for recycling the gaseous stream. When polymer agglomerates are originated within the polymerization reactor, there can be many adverse effects. For example, the agglomerates can disrupt the removal of polymer from the polymerization reactor by plugging the polymer discharge valves. Further, if the agglomerates fall and cover part of the fluidization grid a loss of fluidization efficiency may occur. This can result in the formation of larger agglomerates which can lead to the shutdown of the reactor.